Ultrasound triggering method

ABSTRACT

The invention relates to a triggered ultrasound imaging method for imaging of the myocardium, minimizing the risk of eliciting cardiac arrhythmia. Particularly, the invention is directed to a method of assessing cardiac perfusion. Destruction pulses are triggered such that they fall within the refractory period of the heart, while imaging pulses are triggered at any given time of the ECG cycle, preferably during end-systole.

FIELD OF THE INVENTION

The present invention relates to a method of triggered ultrasoundimaging of the heart of a human or non-human animal subject administeredwith an ultrasound contrast agent wherein the risk of eliciting cardiacarrhythmia is minimized. Further the invention relates to a method ofassessing perfusion of the myocardium.

DESCRIPTION OF RELATED ART

It is well known that contrast agents comprising dispersions of gasmicrobubbles are particularly efficient backscatterers of ultrasound byvirtue of the low density and ease of compressibility of themicrobubbles. For example WO 97/29783 describes such microbubbledispersions. If appropriately stabilised microbubbles may permit highlyeffective ultrasound visualisation of, for example, the vascular systemand tissue microvasculature, often at advantageously low doses.

The following patent documents relate to ultrasound imaging involvingcontrast agent destruction.

It is stated in U.S. Pat. No. 5,425,366 that certain types ofmicroparticulate ultrasound contrast agents, for example gas-containingpolymer microcapsules, may be visualised by colour Doppler techniquesdespite being essentially motionless, e.g. as a result of uptake by thereticuloendothelial system. It is proposed that the relatively highinsonication energy levels associated with colour Doppler investigationscause the microparticles to burst, thereby generating Doppler-sensitivesignals described as “acoustically stimulated acoustic emission”. Itwill be appreciated that since this technique is concerned exclusivelywith detection of essentially motionless contrast agent microparticlesit is inherently inapplicable to measurement of rates of perfusion.Triggering techniques are not described.

WO 98/47533 is based on the finding that ultrasound imaging involvingultrasound-induced destruction or modification of contrast agents may beused to give a measure of tissue perfusion. The method described, knownby various names, e.g. destruction wash-in imaging (DWI), perfusionimaging or triggered replenishment imaging (TRI), and some times calledflash imaging, uses a first high-energy ultrasound pulse or series ofpulses to destroy or discernibly modify a recognisable amount of thecontrast agent within a target region, but rather than employingsubsequent high energy pulses to detect background signals to besubtracted from the first detection sequence the method uses thesubsequent low energy pulses to detect the flow of “fresh” or unmodifiedcontrast agent (and therefore blood) into the target region. Thispermits determination of parameters such as vascular blood volumefraction, mean transit time and tissue perfusion with respect to localvascular state within the target region. The initial high-energy pulseor pulses may, for example, be used to clear a closely defined targetregion of detectable contrast agent so that a sharp front of furthercontrast agent, which is readily detectable and quantifiable byultrasound imaging, then flows into this region. WO 98/47533 mentionsECG-triggering, as one of several techniques for ultrasound-induceddestruction or modification of contrast agents for measurement of tissueperfusion, without any further specifications.

Ultrasound machines capable of DWI, TRI or perfusion imaging, use afirst high-energy ultrasound pulse or series of such pulses, that is,destruction pulses with a high mechanical index (MI), to destroy thecontrast microbubbles e.g. in the myocardium, and then demonstrate thewash-in of microbubbles in the myocardium by imaging using low energypulses (low MI).

Generally, the ultrasound pulses used in perfusion imaging are triggeredto produce discrete single pulses or a sequence of pulses for imaging ordestruction of the ultrasound contrast agent in the vascular system.Technically, triggered imaging is a technique wherein the ultrasoundmachine is synchronized with the echocardiogram (ECG) of the heart, orsimilar cardiac-synchronous signal, or with a clock signal, therebysupplying a triggering signal for initiation of discrete single pulsesor a sequence of pulses. When the ECG signal is used as triggeringrhythm, a single or a given low number of ultrasound pulses or frame(s)is taken at the same predetermined phase of the ECG cycle, either atevery heart beat (trigger interval 1:1) or at a specified interval everyn^(th) heart beat (trigger interval 1:n). When all ultrasound pulsestriggered at every n^(th) heart beat are of the same (high) energylevel, usually with only single pulses, the triggering technique iscalled triggered interval sequencing (TIS). The imaging pulses aretherefore also destruction pulses, both using the same high-energy levelpulses, in TIS. TIS can be used at any given point in the ECG cycle, butis most often initiated during end-systole. Myocardial perfusion isassessed by varying the trigger interval and observing for regionaldifferences in the trigger intervals needed before the maximum contrast“returns” in all myocardial regions. Longer trigger intervals will beneeded in myocardial regions with decreased perfusion compared toregions with normal perfusion. The identical high mechanical index ofdestruction and imaging pulses precludes imaging of contrast build-updue to considerable microbubble destruction. As all ultrasound pulsesserve as both destruction and imaging pulses, the number of high-energytriggered imaging pulses during a clinical procedure of TIS imaging isconsequently high, increasing the risk of eliciting ventricularpremature beats (VPBs). When the first high-energy pulse or sequence ofpulses are part of a destruction imaging sequence at every n^(th) heartbeat, followed by low energy single pulses at every heart beat for up ton−1 heart beats, the triggering technique is called triggeredreplenishment imaging (TRI), as described in principle in WO 98/47533.As high-energy destruction pulse(s) are only triggered every n^(th)heart beat, the number of high-energy triggered imaging pulses during aclinical procedure of TRI is consequently much lower than during aclinical procedure of TIS imaging. Irrespective of triggering technique,second harmonic or pulse inversion or some other non-linear imagingtechnique is usually used during triggered imaging (TIS and TRI).

The heart rhythm is divided into systole and diastole. Systolerepresents the period in which the ventricles contract, while diastolerepresents the period in which the ventricles are relaxed and dilateduring filling with blood. Atrial contractions fill the heart duringend-diastole. The P-wave of the ECG signal represents atrialcontractions and the end of the diastole. The R-wave of the ECG signalrepresents initiation of ventricular contractions during start-systole.The R-wave is the amplitude that is recognized the easiest and mostconsistently by ultrasound machines and by adjusting the trigger delay(time of ultrasound trigger in relation to the R-wave), the actualtrigger point can be adjusted throughout the length and at any point ofthe ECG cycle. Once the trigger delay has been adjusted to the intendedvalue, both TIS and TRI use the same trigger delay for all triggeredpulses, which is an important difference compared with the presentinvention. In humans, end-systolic triggered (EST) imaging usestriggering approximately at the T-wave, about 300 msec after the R-wave,and image the heart during maximal contraction. End-diastolic triggered(EDT) imaging uses triggering approximately at the P-wave. EST is mostoften used clinically during triggered imaging, as the heart is mostcontracted during this phase of the ECG cycle. More of the heart willtherefore be in the imaging sector, the myocardium is thickest and thedegree of shadowing in the ventricle is minimized during EST. In orderto image myocardial perfusion, the contrast agent present in themyocardium has to be destroyed before the wash-in of new microbubblescan be observed. In both TIS and TRI, destruction of the gasmicrobubbles requires high-energy ultrasound pulses (high MI) and whenhigh MI is used, cardiac arrhythmia, such as ventricular premature beats(VPB), may occur in relation to triggering. Trigger-induced arrhythmiaoccurs primarily during end-systolic triggering, which is also the mostrelevant time of the ECG cycle to image during contrast administration.

Generally, triggered ultrasound imaging is primarily used to minimizethe ultrasound destruction of gas microbubbles and to make the visualjudgement of myocardial contrast wash-in easier than during liveimaging. During live imaging, the variations in base-line contrast areoften higher than the contrast build-up during wash-in, hence liveimaging is often little, if at all, useful for assessment of myocardialperfusion. The imaging modes, e.g. second harmonic, pulse inversion,ultra-harmonic and power modulation, used during imaging of ultrasoundcontrast agents take advantage of the non-linear properties of the gasmicrobubbles. However, as second harmonic, pulse inversion andultra-harmonic imaging use a lower transmit frequency, they are oftenmore destructive towards microbubbles than standard B-mode imaging atcomparable mechanical index.

Some ultrasound triggering methods are described by Gardner et al. in2000 IEEE Ultrasonics Symposium Proceedings, 1911-1915, 2000. WhenR-wave-triggered imaging is mentioned, actual triggering of theultrasound pulses and imaging is not done at the R-wave of the ECGcycle, but the ultrasound machine detects the R-wave and the images areacquired at a specified time after the R-wave. All triggering in thepaper is done in end-systole.

Triggered ultrasound imaging of the myocardium has been described by Vander Wouw et al. in J Am Soc. Echocardiogr. 13: 288-294, 2000 and by Vander Wouw et al. in European Heart Journal 20: 683, 1999. Van der Wouw etal. report that trigger-related ventricular premature beats (VPBs) inhumans and animals are elicited during ultrasound imaging usingtriggered interval sequencing (TIS) technique and ultrasound contrastagent administration. Van der Wouw et al report VPBs during EST imaging(triggering at end of T-wave), while no VPBs where observed during EDTimaging (triggering at the interval from P-wave until first deflectionof ECG (Q-wave)) in humans. EST is the preferred option for perfusionimaging as described above. The end-diastolic triggering method used byVan der Wouw is therefore not a suitable option for obtaining perfusiondata. The present invention does not have these limitations.

There is a need for methods that permit better evaluation of coronaryartery disease, and particularly measurements of tissue perfusion.Measurements of blood flow per unit of tissue mass, are of value in, forexample, detection of regions of low perfusion, e.g. as a result ofarterial stenosis. Measurement of cardiac perfusion in order to identifyany myocardial regions supplied by stenotic arteries is of particularimportance. The current invention is directed towards the use ofultrasound contrast agents, i.e. dispersions of microbubbles, in anultrasound imaging triggering method for imaging of the myocardium, andparticularly for perfusion assessments. To achieve this, it is importantto define and refine ultrasound imaging triggering techniques to givemethods that do not result in arrhythmia. A method of triggeredultrasound imaging of the myocardium avoiding or minimizing the risk ofarrhythmia has been sought.

SUMMARY OF THE INVENTION

The following invention provides a method of triggered ultrasoundimaging, for imaging of the heart wherein cardiac arrhythmia, such asVPBs, is minimized.

It has surprisingly been found that a method of triggered ultrasoundimaging of the heart of a human or non-human animal subject administeredwith an ultrasound contrast agent, wherein one high-energy ultrasoundpulse is initiated such that this pulse falls within the refractoryperiod of the heart, is useful. According to the invention, thehigh-energy ultrasound pulse or a sequence of pulses with a high-energylevel are hence triggered at start-systole. The low-energy imagingpulses are best triggered at end-systole.

The main advantage of the invention is that start-systolic triggering ofdestruction pulses according to the invention is unlikely to elicitarrhythmia, such as VPBs. Start-systolic destruction pulses do notaffect the efficacy of subsequent end-systolic imaging pulses, which donot elicit VPBs due to the low energy needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the myocardial membrane action potential recordingthe changes in electrical potential across the membrane.

FIG. 2 illustrates the frequency of VPBs (y) as a function of relativetrigger delay after the R-wave, during infusion of an ultrasoundcontrast agent in dogs, using Triggered Interval Sequencing (TIS)imaging, in relation to the ECG of the heart.

FIGS. 3 and 4 give graphical presentations of existing triggeringtechniques and the non-arrhythmogenic Destruction-Wash-In Imaging(DWI)/Triggered Replenishment Imaging (TRI) and Real-Time PerfusionImaging (RTPI) techniques of the invention.

FIG. 5 shows an example of an ECG-trace captured from videotape.

FIG. 6 gives frequency of VPBs (y) as a function of displayed MI(x)during TIS imaging of dogs administered with an ultrasound contrastagent.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the invention is a method of triggered ultrasoundimaging of the heart of a human or non-human animal subject administeredwith an ultrasound contrast agent wherein one high-energy ultrasoundpulse is initiated such that this pulse falls within the refractoryperiod of the heart.

In a preferred embodiment of the invention the high-energy ultrasoundpulse is repeated to form a sequence of pulses initiated such that thefirst pulse of said sequence falls within the refractory period of theheart.

The method is carried out by administering a subject with an ultrasoundcontrast agent such that this agent is uniformly distributed in theblood pool, and subjected to ultrasound emission, e.g. from anultrasound transducer directed at the heart, or a target region of theheart, in order to destroy or discernibly modify the circulatingcontrast agent. Abrupt termination of the ultrasound emission will givea substantially sharp bolus front as further contrast agent is washedin, and this may be used for assessment of the perfusion in the coronaryarteries. Perfusion may be defined as a measurement of blood volume pertissue weight and unit time. The degree of regional perfusion may beassessed by monitoring the temporal development of contrast effect indifferent regions of tissue upon arrival of the created bolus. Thearrival of contrast to tissue regions of high perfusion is expected totake place earlier than in areas of lower perfusion.

The composition of the heart can basically be divided in pacemaker cellsand normal myocardial cells. The interior of myocardial cells isnormally electrically negative compared to the outside environment, witha resting membrane potential (RMP) of −80 to −90 mV, thatinstantaneously increase to 20-30 mV during depolarisation (FIG. 1). Ifnot excited by external (electrical) stimuli, the increase in RMP isvery slow during depolarization, i.e. in practice almost stable, innormal myocardial cells, while in pacemaker cells the RMP automaticallyincreases such that when the threshold potential (TP) of about −60 mV isreached, the cell depolarizes. Following depolarization to a membranepotential of 20-30 mV, the membrane potential decreases to the RMP in 4phases, as shown in FIG. 1. This figure shows myocardial membranepotential measured by placing an electrode inside a muscle cell and thenrecording the changes in electrical potential (millivolts) that occuracross the membrane over time (seconds). In phase 1, the plateau phaseafter depolarization, the membrane potential is more or less unchanged,while a slow decrease is initiated during phase 2. The membranepotential decreases further in phase 3, where the threshold potential(TP) is passed, before the normal RMP is regained in phase 4. Duringphases 1, 2 and 3, until TP is passed, the myocardial tissue isrefractory to any external stimuli, while the time from TP passage (endof phase 3) and until the resting membrane potential is regained inphase 4 is relatively refractory and is excitable if stimulus issufficiently high. Referring to the ECG-cycle shown in FIG. 2, duringthe normal ECG cycle, the P-wave represents the depolarization of theatria while the isoelectric period between the P-wave and the R-waverepresents the delayed passage of the atrial impulses through theatrioventricular node. The ventricular depolarization is composed of 3main phases, represented by the Q-, R- and S-wave. The Q-wave representsthe first phase of ventricular depolarization (mid- and apical portionsof ventricular septum) while the R-wave represents the propagation ofthe electrical impulse from the sub-endocardial terminations of thePurkinje fibers to the epicardial surface (free walls) of bothventricles. The S-wave represents the depolarization of the muscle fiberat the ventricular basis while the T-wave represents the ion shiftingduring repolarization of the myocardial cells.

When the above membrane potential characteristics of single myocardialcells are applied to the heart, the ventricles are refractory toexternal stimuli during depolarization and the time immediatelyfollowing depolarization. The Q-R-S interval and a short periodthereafter, until threshold potential is regained, is therefore therefractory period of the heart. In FIG. 2 this refractory period isdenoted A. We have now found that it is favourable to use an ultrasoundtriggering method wherein pulses with a high-energy level are initiatedduring this refractory period, because eliciting of any ventricularpremature beats is avoided or minimized. Triggering of ultrasoundhigh-energy pulses during start-systole (denoted A in FIG. 1) isfundamentally different from end-diastolic triggering (P-Q interval), asdescribed by Van der Wouw et al, not only because it is a different partof the ECG cycle and because the destruction pulses used by Van der Wouwalso serve as imaging pulses, but especially because the ventricles arerefractory to external stimuli during start-systole, but not duringend-diastole.

It has surprisingly been found that it is the time of initiation of thedestruction pulse or pulse sequence, rather than the energy level,length, MI, frame-rate or pulse length of the destruction pulsesequence, which determines whether ventricular premature beats areelicited.

In a preferred embodiment of the invention a high-energy pulse orsequence of pulses are initiated at the beginning of the refractoryperiod, i.e. in the Q-R-S interval. Further, the sequence of pulsesshould be continued until just before the first, second or any laterend-systole after initiation, to avoid VPBs.

The high-energy pulses are applied to destruct or discernibly modify theultrasound contrast agent. Preferably the first high-energy ultrasoundpulse coincides with the R-wave of ECG of the heart and more preferablythe high-energy pulse persist throughout the ECG cycle. More preferably,the sequence of high-energy pulses should be adjusted according to theheart rate such that it stops just before a T-wave of the ECG, and mostpreferably, it should stop just before the T-wave of the nextECG-sequence. This will minimize inflow of contrast microbubbles fromend of the destruction pulses until imaging of contrast wash-in isinitiated.

At the same time as the sequence of high-energy pulses stops, furtherlow-energy imaging pulses are preferably initiated. In this embodimentlow-energy pulses are preferably initiated at a T-wave. As described,triggered imaging pulses should be end-systolic (EST), but as they usean ultrasound energy level well below the lowest (ultrasound) energylevel where trigger-related VPBs have been observed, no effects oncardiac rhythm are expected. The high-energy destruction pulsesinitiated at the R-wave are hence followed by low-energy imaging pulsesinitiated at a following T-wave. The imaging pulses are preferablyinitiated at a T-wave, but initiation at other points of the ECG may bedone. The destruction pulses should then end at the same random point.Preferably the imaging pulses are initiated at the first T-waveimmediately following the high-energy pulses or one heartbeatthereafter. Such imaging pulses may be triggered, as shown in FIG. 3Cwith triggering at every heart beat, or may be continuous as shown inFIG. 4E. In order to assess perfusion one would preferably look at asequence of images, but a single parametric perfusion image is also apossibility.

The energy level of the initial ultrasound destruction pulses is highand the pulses should have an energy level or MI high enough to destroyor modify the contrast agent present in the imaging plane. This MI levelwill vary depending on the contrast agent used and the patient imaged,but typically the MI will be of at least 0.2-1.9, and preferably between0.7-1.4. The energy level of the imaging pulses should be low and thepulses should have a mechanical index low enough to image the contrastagent without destroying it or with a minimum destruction. The MI levelwill again vary from agent to agent, but the level will typically be of0.05-1.0, and preferably between 0.1-0.6.

The destruction pulses must be applied long enough to destroy or modifythe contrast agent in the imaging plane and ending as close as possibleto the first low-energy imaging pulse. This could be any duration, fromone single ultrasound frame to several seconds. The destruction pulseswill typically be sent out at the scanners regular frame rate, but sincethe information in these images is generally discarded the frame ratemay be increased at the expense of image quality. The length of thedestruction pulses may also be increased to improve the destruction ofthe ultrasound contrast agent.

Any ultrasound triggering method may be used, subject to that theinitial destruction pulse falls within the refractory period, such aswithin the Q-R-S-interval, or coincides with the R-wave, of the ECG. Thefollowing imaging modes may be used; fundamental (B-mode), second (orany higher) harmonic, sub-harmonic, coherent contrast imaging, coherentpulse sequencing, pulse/phase inversion, ultra harmonic, powermodulation, power pulse inversion, loss of correlation imaging and powercontrast imaging and any combination of these techniques. Preferredtechniques are destruction-wash-in imaging (DWI), triggeredreplenishment imaging (TRI) and real-time perfusion imaging (RTPI).

Myocardial triggering ultrasound techniques can be divided into threemain categories, all using high-energy ultrasound pulses for gasmicrobubble destruction. Current use of these three techniques may allelicit trigger-related arrhythmia such as VPBs. TIS and DWI/TRI triggeraccording to the ECG cycle, while the third, RTPI, trigger manually. DWIand TRI are relatively identical triggering techniques. The destructionpulse sequences are of relatively high mechanical index, while theimaging pulses use low mechanical index. In known methods, bothdestruction pulses and imaging pulses are triggered at the same point inthe ECG cycle, in the end-systole, but different trigger delays of thedestruction and imaging pulses are possible. The trigger intervals ofdestruction pulses and the imaging pulses are variable, with a usualtrigger interval of destruction pulse sequences about 1:8-1:20, whilethe imaging pulses are triggered at every heart beat (1:1). DuringDWI/TRI an initial continuous sequence of high MI destruction pulsesinitiated at a certain time of the ECG cycle destroys the microbubblesand the myocardial contrast. Wash-in of the gas microbubble contrastagent is then imaged at low MI by EST imaging at every heartbeat. Thenumber of high MI EST destruction pulse sequences during a clinicalprocedure is consequently considerably lower with DWI/TRI compared toTIS. However, as the destruction pulse sequence of DWI/TRI in knownmethods is initiated during end-systole like the imaging pulses, thepossibility of trigger-related arrhythmia during the first pulses ofeach destruction pulse sequence exist. Real-Time Perfusion imaging(RTPI) is a technique wherein a sequence of high mechanical indexdestruction pulses is followed by continuous imaging at low mechanicalindex. The operator manually initiates destruction pulses randomly atany given time during the ECG cycle. Initiation during end-systole maytherefore be able to elicit trigger-related arrhythmia.

Existing ultrasound triggering techniques, and particularly the TIStechnique, but also DWI/TRI and RTPI, all include the risks of inducingtrigger-related ventricular premature beats (VPB) and other arrhythmiawhen used for cardiac imaging during infusion of gas microbubblecontrast agents. Trigger-related ventricular premature beats (VPBs) inhumans and animals during ultrasound imaging using the TIS techniquehave been reported in the literature and these findings have beenconfirmed in performed experiments in different animal species. FIG. 2illustrates the probability of ventricular premature beats (y) versustrigger delay (x) during infusion of an ultrasound contrast agent(Sonazoid®), using a TIS technique and a Philips HDI 5000 US machinewith a P3-2 transducer, MI 1.3, see also detailed description inExample 1. At the given time line (approximately 800 msec), a typicalECG of the heart is included in the figure, naming the different wavesof the cycle. The figure shows that the probability of ventricularpremature beats is highest during end-systole, around the T-wave, of theECG. As can be seen from FIG. 2, a relative trigger delay ofapproximately 0.0-0.2 sec, that is the R-S interval plus time needed forrepolarisation, results in no VPBs during TIS, while VPBs occur duringthe remaining ECG cycle (relative trigger delays of approximately0.25-0.95 sec) and particularly during end-systole (0.25-0.35 sec).

Also, during end-systolic triggered (EST) destruction wash-in imaging(DWI) at destruction pulses of MI 1.2 with a Philips HDI 5000 ultrasoundmachine and a P4-2 transducer, very few, but definitely trigger-relatedVPBs, were observed in dogs. While some of these differences in VPBincidence between the TIS and DWI/TRI may be related to inherentdifferences in the nature of the ultrasound transmitted by differenttransducers, trigger-related VPBs are not excluded when high-energyultrasound destruction pulses of DWI/TRI and RTPI are started duringend-systole.

To avoid the induction of trigger-related VPBs during DWI/TRI and RTPIimaging during infusion of gas microbubble contrast agents, we havefound that the destruction pulses and the imaging pulses should beinitiated at different points of the ECG cycle. As VPBs are dependentupon myocardial gas microbubble concentration and the first pulses inthe high MI destruction pulse sequences are elicited during maximalmicrobubble concentration, these first pulses of the destruction pulsesequence are the most likely to elicit VPBs. The chance of VPBs perpulse decreases with every pulse of the destruction pulse sequence dueto continuous microbubble destruction. It has therefore been found thatit is the time of initiation, rather than the length of the destructionpulse sequence, that determines whether VPBs are elicited.

Graphical presentations of the existing techniques compared with thesuggested non-arrhythmogenic triggering DWI/TRI and RTPI techniques ofthe invention are included in FIGS. 3 and 4. In these graphs X denotesdestruction pulses and Y denotes imaging pulses. Graph A of FIG. 3 showsa standard TIS, end-systolic high MI triggering (1:1). The imagingpulses are also destruction pulses. The pulses are applied duringend-systole, i.e. at the T-wave. Graph B of FIG. 3 shows a standardDWI/TRI-technique with end-systolic triggering of high MI destructionpulses (1:8) and end-systolic triggering of low MI imaging pulses (1:1).ATL HDI 5000 and Philips Sonos 5500 are examples of ultrasound machinesthat may be used in both examples. In both these high MI end-systolictriggered techniques there are risks of eliciting arrhythmia.

Graph C of FIG. 3 illustrates a technique of the invention.Non-arrhythmogenic DWI/TRI, R-wave triggering (i.e. start-systolic) ofhigh-energy pulses (1:8) is followed by end-systolic triggering of thelow-energy imaging pulses (1:1). This new type of non-arrhythmogenicDWI/TRI technique, triggering destruction pulses and imaging pulses atdifferent time points in relation to the ECG, is a technical possibilitytoday with the Philips Sonos 5500 ultrasound machine.

Graph D of FIG. 4 shows a standard real-time perfusion imaging technique(RTPI) with a random initiation of high-energy destruction pulsesfollowed by continuous low-energy imaging. An ATL HDI 5000 may be usedin such technique.

Graph E of FIG. 4 illustrates another technique of the invention. Inthis non-arrhythmogenic RTPI technique high-energy destruction pulsesare initiated at the first R-wave, i.e. start-systolic, after aninitiation decided by the operator at a random time point in the ECGcycle, and are then followed by continuous low-energy imaging. The firstline in the X section indicates the initiation by the operator and not adestruction pulse.

In the techniques of the invention, shown in FIGS. 3C and 4E, thedestruction pulses and the imaging pulses are initiated at differentpoints in the EGC. The high-energy destruction pulses are initiated atthe R-wave (start-systolic), while the low-energy imaging pulses areinitiated at the T-wave (end-systolic). The FIGS. 3A, 3B and 4D areshown for comparison.

The preferred time delay between i.v. injection of the ultrasoundcontrast agent and start of data acquisition (destruction/imaging) istypically in the order of tens of seconds following a bolus injection.For an i.v. infusion of microbubbles the preferred time delay is thetime required to reach an approximate steady state of contrastenhancement of the blood. A stable and consistent microbubbleconcentration throughout the DWI/TRI, RTPI and TIS techniques is aprerequisite for assessing microbubble wash-in as an indication ofcardiac perfusion. Start of data acquisition should therefore not bestarted until the microbubble concentration is stable, usually 1-10minutes after start of microbubble infusion.

In principle any free flow ultrasound contrast agent may be used in themethod of the invention, subject only to the requirement that the sizeand stability of the contrast agent moieties are such that they arecapable, following intravenous injection, of passing through the lungcapillaries and generating responses in the left ventricle of the heartand the myocardial circulation. Contrast agents which comprise or arecapable of generating gas microbubbles are preferred since microbubbledispersions, if appropriately stabilised, are particularly efficientbackscatterers of ultrasound by virtue of the low density and ease ofcompressibility of the microbubbles. Ultrasound contrast agentscomprising a vector having affinity for a biological target are alsoenclosed. The ultrasound contrast agents described by the followingpatent families are relevant for use in the method of the invention, forpurposes of illustration and not of limitation: WO97/29783, WO92/17212,WO97/29782, EP 554213, WO-9516467, EP474833, EP 619743, U.S. Pat. No.5,558,854, WO92/17213.

Examples of ultrasound contrast agent that may be used according to theinvention are, for purposes of illustration and not of limitation,Optison®, Levovist®, Definity®, Imagent®, Sonovue®, Echogen®, Sonogen®and Sonazoid®

A variety of acquisition ways may be used to detect and quantifyinflowing contrast agent following the initial ultrasound destruction,e.g. to generate a perfusion related image displaying a time-relatedmeasure of in-flowing contrast agent within the target region andthereby permitting discrimination between areas of different perfusion.The desired image may be obtained from analysis of individual scanlinesor on a frame by frame basis; the former may be advantageous in areaswith high rates of perfusion in order to obtain sufficient numbers ofsamples to discriminate areas with different perfusion, whereas thelatter may be preferred in areas with low rates of perfusion.

The imaging method of the invention may be used in measurement ofcardiac perfusion, and this forms a further embodiment of the invention.With the triggered ultrasound imaging method of the invention myocardialperfusion assessments, making the visual judgement of myocardialcontrast wash-in easier, can be performed with no, or minimal, risk ofeliciting ventricular premature heart beats. A further embodiment ishence a method of measuring or assessing myocardial perfusion in a humanor non-human animal subject comprising administering an effective amountof an ultrasound contrast agent to the subject, and subjecting a targetregion of the myocardium with a high-energy ultrasound pulse initiatedsuch that this pulse falls within the refractory period of the heart ofthe subject.

In such myocardial perfusion assessment the high-energy ultrasound pulseis preferably repeated to form a sequence of pulses initiated such thatthe first pulse of said sequence falls within the refractory period ofthe heart.

Use of an ultrasound contrast agent in a method as described is afurther aspect of the invention.

Use of an ultrasound contrast agent in the manufacture of animage-enhancing composition for administration to the vascular system ofa human or non-human animal subject in order to measure or assess theperfusion of the myocardium in a method wherein one high-energyultrasound pulse is initiated such that this pulse falls within therefractory period of the heart is a further aspect. The embodimentsdescribed for the method of the invention is included in such aspect.

Preferably, the subject has been preadministered with an ultrasoundcontrast agent before the method of the invention is performed. In afurther aspect the invention provides a method of ultrasound-induceddestruction or modification of an ultrasound contrast agentpreadministered to a human or non-human animal body, subjecting a targetregion of the heart of the body with one high-energy ultrasound pulseinitiated such that this pulse falls within the refractory period of theheart, enabling destruction or modification of the contrast agent with aminimized risk of eliciting arrhythmia. Such method may further includethe embodiments described for the triggered ultrasound imaging method,including repeated high-energy pulses, followed by additional low-energypulses in order to create an ultrasound image.

The invention may be accomplished by modifying the existing software inthe ultrasound machines by implementing facilities enabling automatictriggering of high mechanical destruction pulses and low mechanicalimaging pulses at different time-points in relation to the ECG. Thesoftware should allow for automatic beat per beat adjustments ofdestruction pulse sequence length according to heart rate. The abilityto trigger destruction pulses and imaging pulses at different timepoints in relation to the ECG is today technically possible with theSonos 5500, but the destruction pulse sequence length is currently notautomatically adjusted according to heart rate variations.

The ultrasound contrast agent could be administered as a bolus injectionor by infusion, when performing the method of the invention. Preferablythe contrast agent is administered by infusion. By applying high-energypulses according to the invention, a local bolus effect is created,enabling assessment of the perfusion. Using the method in combinationwith bolus administration may be of interest if wanting to startdestruction, in order to come back to baseline, at the R-wave withoutfurther assessment of wash-in.

While the preferred embodiment of the present invention has been shownand described, it will be obvious in the art that changes andmodifications may be made without departing from the teachings of theinvention. The matter set forth in the foregoing description andaccompanying drawings is offered by way of illustration only and not asa limitation. The actual scope of the invention is intended to bedefined in the following claims when viewed in their proper perspectivebased on the prior art.

EXAMPLES

In vivo studies were performed to better understand what parametersaffect the occurrence of cardiac arrhythmias when performing triggeredcontrast echocardiography. A successful model was established, differentultrasound scanners and imaging parameters were tested and an imagingprotocol for minimising the risk of trigger induced arrhythmias issuggested.

Example 1

To investigate this phenomenon, to see if a triggered imaging protocolthat did not induce VPBs could be developed, Triggered ContrastEchocardiography (TCE) was conducted in mongrel dogs (Body weight: 9-32kg, mean: 22 kg). The animals were anaesthetized with fentanyl andpentobarbital and mechanically ventilated with a respirator using roomair. The protocol was approved by the local ethical committee and allprocedures were terminal and performed according to current guidelinesand regulations.

Three ultrasound scanners with four cardiac transducers were used. Thescanners were a Philips HDI 5000 with P3-2 and P4-2 transducers(Andover, Mass., USA), a Siemens Sequoia 512 with 3V2c transducer(Mountainview, Calif., USA) and a Philips Sonos 5500 with S3 transducer(Andover, Mass., USA). Various imaging modes, MIs and triggeringprotocols were tested during infusion of Sonazoid™ (Amersham Health).The infusion rate was 2-5 ml Sonazoid™ per hour (2-7 times clinicaldose). The dose was adjusted for maximum contrast enhancement withoutsignificant shadowing. Except for a shorter focal depth and a modifiedinfusion procedure, all US machinery and protocols were identical toprocedures used in a clinical setting.

Standard 3-lead ECG connections were placed on the extremities and thebest ECG lead chosen for trigger signal and display on the US machine.All US images and associated ECG tracings were recorded continuously onvideotape. Imaging was performed through the chest wall with thetransducer mechanically fixated. The imaging plane was transversemid-papillary. Each transducer was tested at max MI during secondharmonic imaging (SHI), triggered in end systole-every eighth heartbeat(TIS technique). Other settings were: minimum image depth and a singlefocal point 4 cm deep. Ultraharmonics and Power modulation was tested inaddition to SHI with the S3 transducer. With the P4-2 transducer theTriggered replenishment imaging (TRI) protocol with pulse inversion(destruction pulse MI: 0.8, 1.0 and 1.2, imaging pulse MI: 0.4) wastested in addition to SHI. With the P3-2 transducer the effect ofchanging the trigger delay after the R-wave of the ECG complex, thetriggering interval and the MI was also studied.

The transducer settings were kept constant for 25 to 200 triggeringpoints when acquiring VPB frequency data. The shortest observation timewas used when testing the effect of variations in trigger delay, sincethe variation from one tested setting to the next was small (40 ms inthe most sensitive area). The longest observation time was used whencomparing transducers and imaging modes.

Results:

VPBs were observed in all animals after careful positioning of thetransducer. The optimal imaging plane for VPB studies could not beidentified by anatomical structures alone, but had to be guided by TCEwith the settings most likely to elicit VPBs. When switchingtransducers, careful comparison with the previously videotaped imagingplane was performed to get the least possible variations in the imagingplane.

FIG. 5 shows an example of an ECG-trace captured from video tape. TheVPB displays an abnormal QRS complex right after the trigger pointindicated by the small vertical line, marked with an arrow.

FIG. 6 gives frequency of VPBs (y) as a function of displayed MI (x).1:8 ES triggering was used with the P3-2 transducer in HPEN mode. Meanresults from six animals.

FIG. 2 gives the frequency of VPBs (y) as a function of relative triggerdelay after the R-wave (x). A sample ECG-trace is drawn above the graphfor reference. 1:5 triggering was used to minimise the observation timewhen testing the effect of variations in trigger delay after the R-wave.This was tested in four animals with the P3-2 at an MI of 1.3 and atriggering interval of 1:8. The results are plotted in FIG. 2 as afunction of triggering delay relative to the R-R interval (determined bythe heart rate). 0 indicates the peak of the first R-wave, approximately0.3-0.4 is end-systole and 1 is the peak of the next R-wave. There werelarge variations in the frequency of arrhythmias for each animal, but itwas found that none of the animals had any VPBs in start-systole (A),when the heart is in a refractory phase.

No adverse events following the VPBs were observed in any of theanimals.

Destruction Wash-In Imaging:

As destruction wash-in imaging is starting to be a more used method forperfusion imaging with ultrasound contrast agents, the TRI protocol withthe P4-2 was tested in three animals, with TIS using the P3-2 as apositive control. The results are shown in Table 1. Only three VPBs weretriggered by more than 9000 destruction pulses and 68000 imaging pulsesduring TRI. TIS with the P3-2 transducer gave a VPB frequency two ordersof magnitude higher. All triggering was done in end-systole. See FIG. 3Afor an illustration of the TIS technique and FIG. 3B for an illustrationof the TRI technique. Start-systolic triggering, as given in FIG. 3C,would further have reduced the number of VPBs. TABLE 1 Triggering Trans-Trigger # of trigger # of VPBs/ technique ducer interval MI events VPBstrigger TRI destruction P4-2 1:8 0.8 3155 1 3.2 · 10⁻⁴ pulses 1.0 3348 00 1.2 3335 2 6.0 · 10⁻⁴ TRI imaging P4-2 1:1 0.4 68862 0 0 pulses TISP3-2 1:8 1.3 544 24 0.044 1.2 302 14 0.046

1. Method of triggered ultrasound imaging of the heart of a human ornon-human animal subject administered with an ultrasound contrast agentwherein one high-energy ultrasound pulse is initiated such that thispulse falls within the refractory period of the heart.
 2. Method oftriggered ultrasound imaging as claimed in claim 1 wherein thehigh-energy ultrasound pulse is repeated to form a sequence of pulsesinitiated such that the first pulse of said sequence falls within therefractory period of the heart.
 3. Method as claimed in claim 1, whereinthe first high-energy ultrasound pulse falls within the Q-R-S intervalof the electrocardiogram of the heart.
 4. Method as claimed in claim 1,wherein the first high-energy ultrasound pulse coincides with the R-waveof the ECG of the heart.
 5. Method as claimed in claim 1, wherein inaddition low energy imaging pulses are initiated after the high-energyultrasound pulse or sequence of pulses.
 6. Method as claimed in claim 5wherein the low energy imaging pulses are initiated at or around aT-wave of the ECG of the heart.
 7. Method as claimed in claim 1, whereinthe ultrasound technique used is selected from destruction-wash-inimaging, triggered replenishment imaging and real-time perfusionimaging.
 8. Method as claimed in claim 1, used in assessments ofmyocardial perfusion.
 9. Use of an ultrasound contrast agent in a methodas claimed in any of the preceding claims.
 10. Use of an ultrasoundcontrast agent in the manufacture of an image-enhancing composition foradministration to the vascular system of a human or non-human animalsubject in order to measure or assess the perfusion of the myocardium ina method wherein one high-energy ultrasound pulse is initiated such thatthis pulse falls within the refractory period of the heart.
 11. Methodof ultrasound-induced destruction or modification of an ultrasoundcontrast agent preadministered to a human or non-human animal body,subjecting a target region of the heart of the body with one high-energyultrasound pulse initiated such that this pulse falls within therefractory period of the heart, enabling destruction or modification ofthe contrast agent with a minimized risk of eliciting arrhythmia.